This invention relates generally to a method and system of avoiding mechanical resonance and thereby reducing the audible noise in mechanical system such as HVAC systems. Specifically, the frequency of the torque ripple is shifted to avoid mechanical resonance of a blower assembly being driven by a brushless DC machine.
Torque ripple in conventionally excited brushless DC motors is related to the flatness of the torque waveform of the motor, which in turn depends on the back EMF and current waveforms. A single-phase brushless DC motor inherently develops relatively higher torque ripple magnitude than it's three-phase counterpart. Such torque ripple results in mechanical vibration of the mechanical system resulting in undesired audible noise. It will be appreciated that the audible noise generated depends upon several factors including mechanical system resonance frequency, magnitude and frequency of the torque ripple developed by motor. The frequency of torque ripple is typically of order 50 Hz-250 Hz, which is in vicinity to the mechanical system resonance frequency.
Various other methods have been proposed in the past for torque ripple reduction of single-phase electrically commutated machines (ECM) which fall mainly in either following approaches:                (A) Advancing the commutation angle of motor drive circuit, which reduces negative torque as well as peak-to-peak torque ripple;        (B) Injecting a constant current into the motor.        
Advancing the commutation angle requires sophisticated controls and additional feedback sensors and increases motor current. Injecting a constant current decreases efficiency and increases heat dissipation. Although these methods would reduce the torque ripple magnitude they do not address the fact that the frequency of the torque ripple still remains below the mechanical assembly resonant frequency. Hence the torque ripple produced still results in audible noise. Therefore, what is needed in the art is a method for shifting the spectral content of the torque ripple and to make it flat to avoid the mechanical assembly excitation.
The above discussed and other features and advantages of the embodiments will be appreciated and understood by those skilled in the art from the following detailed description and drawings.